The present invention is related to braking systems for use on air driven centrifuges and more particularly is related to a means for providing a complete stop to the rotation of the rotor on its cushion of air.
During the centrifugation operation of an air driven centrifuge the rotor typically reaches extremely high rotational speeds in the neighborhood of 150,000 to 200,000 r.p.m.'s. Because the rotor is operating on a virtually friction free cushion of air, the rotor will continue to rotate at very high rotational speeds for a period of time subsequent to the operation of the air driving jets. The cushion of support air is supplied by a support stream of air, which tends in some centrifuge arrangements to exert a slight continued rotational effect on the rotor due to the interaction of the rotor flutes with the supporting air stream.
Once a fluid mixture has been subjected to centrifugation and certain constituents of the mixture have been separated, it is extremely important that the rotor not be subjected to any unstable or jerking motions during its deceleration to a stop. Otherwise, the separated constituents may become remixed, requiring another centrifugation operation. One source of possible unwanted jerking motion is making the rotor stop too quickly or almost instantaneously. Consequently, it is desirable to have the rotor slow somewhat gradually before coming to a stop.
Some prior art methods of stopping the rotor utilize mechanisms which physically contact the rotor, resulting in the frictional wearing of the parts and requring maintenance as well as introducing an additional parameter or effect upon the rotor which may contribute to additional vibrations or wobbling in the rotor that could remix the centrifuged sample.
In certain rotor arrangements, such as shown in my copending application filed on an even date herewith entitled Centrifuge Rotor for Separating Phases of a Liquid, the rotor has a chamber in which the separated constituents are sealed from the remainder of the mixture to alleviate the concern of remixing. Therefore, the need for a gradual deceleration with such a rotor is eliminated, permitting extremely quick stoppage of the rotor. In one quick braking apparatus the flow of the driving air jets is reversed to counter the rotational speed of the rotor to stop the rotational speed of the rotor, but the incident effects of the reversing air flow on the flutes of the rotor prevent the ability to completely stop the rotational movement. Another approach is set forth in a copending application by Douglas H. Durland, George N. Hein, Jr. and Robert J. Ehret filed on even date herewith entitled Eddy Current Brake for an Air Centrifuge. Here a magnetic field is utilized to create eddy currents within the rotor, causing it to quickly decelerate and almost completely stop. However, the rotor will continue to rotate slightly at approximately five to 10 revolutions per second, because there is generally always some slight turning effect placed on the rotor by the supporting air flowing over the rotor flutes.
In order to facilitate convenient removal of the rotor from the centrifuge it is desirable to bring the rotor to a complete stop while it is being supported on a cushion of air. Otherwise, since the rotor subsequent to deceleration continues to rotate at approximately five to 10 revolutions per second due to the supporting air, stopping of the supporting air with the resulting spinning contact of the rotor onto the rotor seat will tend to cause the rotor to thrash around within the rotor seat. If the centrifuged sample within the rotor is susceptible to remixing, the sudden contact of the rotor with the rotor seat will cause an undesired remixing of the sample.
Therefore, a rotor which is riding on a cushion of air during deceleration must be brought to a complete stop subsequent to either a gradual or quick deceleration before turning off the supporting air and allowing the rotor to contact the rotor seat.